Add bus.Peer struct and explicit Peer/MessageID fields to InboundMessage,
replacing the implicit peer_kind/peer_id/message_id metadata convention.
- Add Peer{Kind, ID} type to pkg/bus/types.go
- Extend InboundMessage with Peer and MessageID fields
- Change BaseChannel.HandleMessage signature to accept peer and messageID
- Adapt all 12 channel implementations to pass structured peer/messageID
- Simplify agent extractPeer() to read msg.Peer directly
- extractParentPeer unchanged (parent_peer still via metadata)
Hoshina